The present invention relates to a resonator constituting a radio frequency filter and the like, used for a radio frequency circuit device of a mobile communication system and the like.
Conventionally, a radio frequency communication system indispensably requires a radio frequency circuit element basically constructed of a resonator, such as a radio frequency filter. As a resonator for a low-loss radio frequency filter, often used is a dielectric resonator including a dielectric secured in a conductor shield.
FIGS. 19A and 19B are a perspective view and a cross-sectional view, respectively, of a conventional dielectric resonator 503 often used for a low-loss dielectric filter, which operates in a TE01xcex4 mode as the base mode. The dielectric resonator 503 includes a cylindrical dielectric 501 and a cylindrical case 502 surrounding the dielectric 501 with a space therebetween. The dielectric 501 is mounted on a support and connected to the bottom portion of the case 502 via the support. The ceiling of the case 502 is apart from the top surface of the dielectric 501 by a given distance, and the sidewall (cylindrical portion) of the case 502 is apart from the cylindrical face of the dielectric 501 by a given distance.
Note that the case 502 is actually constructed of a case body and a lid as shown in FIG. 20 although it is shown in a simplified form in FIGS. 19A and 19B.
The above resonator using a TE mode (hereinafter, referred to as a xe2x80x9cTE-mode resonatorxe2x80x9d) is superior to resonators using other modes in that it is small in loss and exhibits a good Q value, but has a disadvantage of being large in volume. Therefore, when a small resonator is desired, a resonator using a mode other than the TE mode as the base mode is used in some cases at the expense of the Q value characteristic to some extent.
FIG. 20 is a cross-sectional view of a radio frequency filter 530 having a resonator using a TM mode (hereinafter, referred to as a xe2x80x9cTM-mode resonatorxe2x80x9d) that is considered a promising candidate for downsizing implementation. The resonator shown in FIG. 20 uses a TM mode called a TM010 mode among the other TM modes.
Referring to FIG. 20, the radio frequency filter 530 includes a cylindrical dielectric 540 and a case 531 composed of a case body 532 for housing the dielectric 540 and a lid 533. The case body 532 and the lid 533 are tightened together with bolts 535 so that the bottom surface of the lid 533 is in contact with the top face of the sidewall of the case body 532. The bottom surface of the lid 533 and the top surface of the bottom portion of the case body 532 are in contact with the top and bottom surfaces of the dielectric 540, respectively. In other words, the dielectric 540 is sandwiched between the lid 533 and the case body 532. The sidewall (cylindrical portion) of the case body 532 concentrically surrounds the dielectric 540 with a space therebetween. An input coupling probe 536 for input coupling with the dielectric 540 and an output coupling probe 537 for output coupling with the dielectric 540 are formed at the bottom portion of the case body 532.
However, it was found that the TM010 mode resonator shown in FIG. 20 failed to provide expected filter characteristics when it was actually prototyped. The present inventors consider the reason for this failure is as follows.
In the TE mode (TE01xcex4 mode) resonator shown in FIGS. 19A and 19B, most of electromagnetic energy is confined within the dielectric, and only a small amount of radio frequency current flows to the side portion of the case 502. However, in the TM mode resonator shown in FIG. 20, a radio frequency induced current flows in the side portion of the case body 532 in a direction parallel to the axial direction. Therefore, conductor loss comparatively largely influences the TM mode resonator. In particular, a large current flows across the corner at which the sidewall of the case body 532 and the lid 533 meet forming a connection Rcnct. If contact failure occurs at the connection Rcnct during the actual assembly of the resonator 530, this will presumably cause large deterioration in Q value and instability of operation. In addition, it has been found that if a gap exists between the top or bottom surface of the dielectric 540 and the lid 533 or the case body 532 due to size errors of components during the manufacture and the like, the resonant frequency sharply increases, and this possibly causes instability of operation. In particular, in the case of assembling a plurality of resonators to construct a filter, it is required to accurately fix the resonant frequency of the plurality of resonators. Therefore, in order to obtain desired filter characteristics while being free from instability of operation, considerably complicated work is presumably required.
In construction of a radio frequency filter using either type of resonator, the TE mode resonator or the TM mode resonator, the following three functions are important: that is,
(1) securing intense input/output coupling having a desired fractional bandwidth;
(2) having a resonant frequency adjusting mechanism that can reduce deterioration in the Q value of the resonator and also easily secure a wide frequency adjustable range; and
(3) having an inter-stage coupling degree adjusting mechanism that can easily secure a wide coupling degree adjustable range in the case of constructing a multi-stage radio frequency filter having a plurality of resonators. It is desired to implement a radio frequency filter having these functions.
A first object of the present invention is providing a dielectric resonator and a radio frequency filter that are small in size, have a simple structure, and operate stably.
A second object of the present invention is providing a radio frequency filter having the functions (1) to (3) described above.
The first resonator of the present invention includes: a columnar dielectric; and a shielding conductor surrounding the dielectric, the resonator using a resonant mode causing generation of a current crossing a corner of the columnar dielectric, wherein the shielding conductor is formed in direct contact with the surface of the dielectric.
With the above construction, the corner of the resonator is constructed of the continuous shielding conductor. Therefore, even in the resonator using a TM mode in which a radio frequency induced current flows over the side face of the column parallel to the axial direction of the column and the end face thereof orthogonal to the axial direction, good conduction is secured, and stability against vibration and the like is secured. Thus, deterioration in Q value and instability of operation are suppressed, and the characterbility of operation are suppressed, and the characteristics of the TM mode resonators of being able to be downsized and having a good Q value can be provided.
The dielectric may include a center portion and an outer portion covering at least part of the center portion, and the dielectric constant of the center portion is higher than the dielectric constant of the outer portion. This reduces conductor loss particularly at the cylindrical portion, and thus improves the unloaded Q value.
The columnar dielectric may be in a shape of a cylinder or a square pole. This facilitates the manufacture.
The shielding conductor may be a metallized layer formed on the surface of the dielectric. This provides high adhesion to the dielectric, and thus the effect is significant.
The second resonator of the present invention includes: a dielectric; and a case for housing the dielectric, wherein part of the case is constructed of conductive foil, and the conductive foil partly shields the dielectric electromagnetically.
With the above construction, the conductive foil is formed at a position such as a seam of the case in which electromagnetic shielding is unstable, to secure the electromagnetic shielding function. This stabilizes the operation characteristics of the resonator.
Preferably, the case includes a first portion and a second portion, the conductive foil is interposed between the first portion and the second portion, and the dielectric is electromagnetically shielded by the first portion and the conductive foil. With the conductive foil interposed at the connection between the first and second portions, vibration can be absorbed by the conductive foil if generated between the first and second portions, thereby suppressing deterioration in connection between the first and second portions. This suppresses deterioration in Q value and improves the stability of operation.
Preferably, the case includes a first portion and a second portion, the conductive foil is interposed between the dielectric and the second portion of the case, and the dielectric is sandwiched between the first portion and the second portion of the case. This nicely sustains the contact between the dielectric and the conductive foil, and thus suppresses occurrence of problems such as sharp increase in resonant frequency.
The resonator may further include an elastic layer interposed between the conductive foil and the second portion. This provides the effect of absorbing vibration more significantly.
The resonant mode of the resonator may include a TM mode. This nicely secures the conduction between the first portion and the conductive foil.
The third resonator of the present invention includes: a dielectric having a hole; a case surrounding the dielectric; and a conductor rod inserted into the hole of the dielectric, the insertion depth of the conductor rod being variable, wherein a resonant frequency is adjusted with the insertion depth of the conductor rod into the hole.
With the above construction, the resonant frequency can be easily adjusted over a wide range without deteriorating the unloaded Q value in a practical level.
The first radio frequency filter of the present invention includes: a dielectric; a conductor member for electromagnetically shielding the dielectric; a conductor probe extending from a portion of the conductor member through a space defined by the conductor member to reach another portion of the conductor member, for coupling the dielectric with an external input signal or an external output signal.
With the above construction, intense input/output coupling is obtained between the dielectric and an external signal even when the radio frequency filter is downsized. This makes it possible to provide a small filter having a good Q value.
The second radio frequency filter of the present invention is a radio frequency filter having a columnar resonator using a resonant mode causing generation of a current crossing a corner, the resonator including: a dielectric; and a shielding conductor surrounding the dielectric formed in direct contact with the surface of the dielectric.
With the above construction, the corner of the resonator is constructed of the continuous shielding conductor. Therefore, even in the resonator using a TM mode in which a radio frequency induced current flows over the side face of the column parallel to the axial direction of the column and the end face thereof orthogonal to the axial direction, good conduction is secured, and stability against vibration and the like is secured. Thus, it is possible to provide a radio frequency filter that can suppress deterioration in Q value and instability of operation, and uses the characteristics of the TM mode resonators of being able to be downsized and having a good Q value.
The third radio frequency filter of the present invention is a radio frequency filter having a resonator, the resonator including: a dielectric; and a case for housing the dielectric, wherein part of the case is constructed of conductive foil and the conductive foil partly shields the dielectric electromagnetically.
With the above construction, the conductive foil is formed at a position such as a seam of the case in which electromagnetic shielding is unstable, to secure the electromagnetic shielding function. Thus, a radio frequency filter having a resonator with stable operation characteristics can be provided.
The fourth radio frequency filter of the present invention is a radio frequency filter having a resonator, the resonator including: a dielectric having a hole; a case surrounding the dielectric; and a conductor rod inserted into the hole of the dielectric, the insertion depth of the conductor rod being variable, wherein a resonant frequency is adjusted with the insertion depth of the conductor rod into the hole.
With the above construction, it is possible to provide a radio frequency filter having a resonator of which the resonant frequency can be easily adjusted over a wide range without deteriorating the unloaded Q value in a practical level.
The fifth radio frequency filter of the present invention is a radio frequency filter having a plurality of resonators at least including an input-stage resonator having a dielectric and receiving a radio frequency signal from an external device and an output-stage resonator having a dielectric and outputting a radio frequency signal to an external device. The radio frequency filter includes: a case surrounding the plurality of resonators for electromagnetically shielding the respective resonators; a partition formed between resonators of which electromagnetic fields are coupled with each other among the plurality of resonators; an inter-stage coupling window formed at the partition; and an inter-stage coupling degree adjusting member made of a conductor rod for adjusting the area of the inter-stage coupling window.
Thus, in the construction of a multi-stage radio frequency filter having a plurality of resonators, it is possible to provide an inter-stage coupling degree adjusting mechanism that is simple and has a wide coupling degree adjustable range, between adjacent ones of the plurality of resonators.